Accent on Imperative forms

I had as a rule that imperative forms were stressed as the 2nd. person singular is, sometimes I see that like the 3rd. person plural. No difference. But I find that sometimes this rule is not so regular. Finally I have found that are exceptions to this rule that must be kept in mind. I ask: There is some, though not very strict rule for this matter, Accent of Imperative?
Thanks